Anti-reflection coatings have conventionally been produced by a physical vapor deposition method such as vacuum vapor deposition, sputtering, ion plating, etc. Because single-layer anti-reflection coatings are designed to have smaller refractive indexes than those of substrates, coating materials having extremely small refractive indexes are desired for anti-reflection coatings formed on substrates having small refractive indexes. Among coatings formed by a physical vapor deposition method, an MgF2 coating has a minimum refractive index of 1.38. However, MgF2 does not have a refractive index of 1.2-1.25, which is ideal for anti-reflection coatings for glass lenses having a refractive index of about 1.5. An anti-reflection coating having a refractive index of 1.2-1.25 exhibits reflectance of less than 1% in a visible-light region having a wavelength of 400-700 nm. However, the anti-reflection coating of MgF2 having a refractive index of 1.38 fails to exhibit reflectance at an ideal level, although less tan 2%.
Liquid-phase methods such as a sol-gel method, an SOG method, etc. are recently used to produce anti-reflection coatings. The liquid-phase methods are advantageous in producing anti-reflection coatings without needing a large apparatus unlike the physical vapor deposition method, and without exposing substrates to high temperatures. However, anti-reflection coatings produced by the liquid-phase methods have refractive indexes near 1.37 at minimum, which is substantially on the same level as those obtained by the physical vapor deposition methods, and there are no large differences in anti-reflection characteristics therebetween. Accordingly, in both methods, low-refractive-index materials and high-refractive-index materials should be laminated to form multi-layer coatings, to have reflectance of less than 1% in a visible-light-wavelength region.
Known as a material having a smaller refractive index than that of magnesium fluoride is silica aerogel. Silica aerogel having a density of 0.01 g/cc or less and a refractive index of less than 1.1 can be formed by preparing a wet silica gel by the hydrolysis of an alkoxysilane, and drying it by an ultra-critical fluid of carbon dioxide, water, an organic solvent, etc. However, this method is highly costly because it needs lengthy steps using an ultra-critical-drying apparatus. In addition, the silica aerogel produced by this method is brittle because of extremely small toughness, failing to be put into practical use.
U.S. Pat. No. 5,948,482 describes a method for producing a thin silica aerogel coating by using a material obtained by (a) preparing a sol containing SiO2, (b) aging it to a gel, (c) modifying its surface with organic groups, and (d) subjecting the organically modified gel to an ultrasonic treatment. This method can produce a silica aerogel coating having a porosity of 99% or more, thus a low refractive index. However, the silica aerogel coating produced by this method has small mechanical strength and poor scratch resistance.